1. Field of the Invention
This invention relates to a process for producing xylenes from reformate by methylating benzene and/or toluene present in the reformate to produce xylenes.
2. Description of the Prior Art
Most aromatics production is based on the recovery of aromatics derived from catalytic reforming of naphtha. That process, using a feed containing a C6+ hydrocarbons, typically produces a reformate comprised of C6-C8 aromatics along with paraffins and heavier aromatics.
Aromatics can also be produced by the dehydrocyclo-oligomerization of C2-C5 aliphatic hydrocarbons. That process typically produces a product comprised of benzene, toluene, xylenes, C5+ paraffins, C4− light paraffins, olefins, and unreacted C2-C5 aliphatic hydrocarbons.
Another technique for producing aromatics involves the cracking of hydrocarbons such as by steam cracking or catalytic cracking. That process typically produces a product comprised of benzene, toluene, xylenes, C6+ paraffins, and other hydrocarbons.
The aromatics present in the reformate stream from a reformer or cracker will depend on the composition of the feedstock to the reformer or cracker, the type of reformer or cracker, and the operating conditions of the reformer or cracker. Normally, the aromatics present in the reformate stream will comprise benzene, toluene, a near equilibrium mixture of xylenes, ethylbenzene, and a mixture of nominally of C9-C10. Products of the reformate having the most value are benzene and xylenes. Of the xylene isomers, i.e., ortho-, meta- and para-xylene, the para-xylene is of particular value as a large volume chemical intermediate in a number of applications, such as the manufacture of terephthalic acid, which is an intermediate in the manufacturer of polyester.
The reformate is usually sent to an aromatics recovery complex where it undergoes several processing steps in order to recover high value products, e.g., xylenes and benzene, and to convert lower value products, e.g., toluene, into higher value products. For example, the aromatics present in the reformate are usually separated into different fractions by carbon number; e.g. benzene, toluene, xylenes, and ethylbenzene, etc. The C8 fraction is then subjected to a processing scheme to make more high value para-xylene. Para-xylene is usually recovered in high purity from the C8 fraction by separating the para-xylene from the ortho-xylene, meta-xylene, and ethylbenzene using selective adsorption or crystallization. The ortho-xylene and meta-xylene remaining from the para-xylene separation are isomerized to produce an equilibrium mixture of xylenes. The ethylbenzene is isomerized into xylenes or is dealkylated to benzene and ethane. The para-xylene is then separated from the ortho-xylene and the meta-xylene using adsorption or crystallization and the para-xylene-deleted-stream is recycled to extinction to the isomerization unit and then to the para-xylene recovery unit until all of the ortho-xylene and meta-xylene are converted to para-xylene and recovered.
Toluene is typically recovered as a separate fraction and then may be converted into higher value products, e.g., benzene and/or xylenes. One toluene conversion process involves the disproportionation of toluene to make benzene and xylenes. Another process involves the hydrodealkylation of toluene to make benzene.
Both toluene disproportionation and toluene hydrodealkylation result in the formation of benzene. With the current and future anticipated environmental regulations involving benzene, it is desirable that the toluene conversion not result in the formation of significant quantities of benzene.
Xylenes can be produced by the methylation of toluene. Such a process is disclosed in U.S. Pat. No. 3,965,207. One advantage of producing xylenes by this process is that the xylenes production does not result in the formation of benzene by-product.
In the past when it was desirable to methylate toluene, toluene present in the reformate has usually been first separated from the other hydrocarbons present in the reformate, such as by fractionation and extraction, before entering a methylation reactor. Such a separation requires substantial capital investment in equipment, e.g., heat exchangers, high pressure separator, fractioners, etc. In addition, the reformate leaving the reformer is at elevated temperature and highly suitable for further conversion. With the separation of toluene from the reformate, the recovered toluene must be heated again to conversion temperatures.